Science Update Winter 1998 — In the 1790s, Luigi Galvani first applied electrical current to a frog's leg, stimulating the leg muscles to contract. The 1970s saw the first reported use of electrical stimulation of the legs to allow people with spinal cord injury (SCI) to stand and walk. It was widely hoped that advances in mechanical control systems to restore effective locomotion after SCI would soon follow.

Since then, the applications of these new technologies after SCI have been limited. Problems have included limitations on walking speed, rapid onset of muscle fatigue, and the high energy demands of system prototypes. Designers strive to overcome these limitations, and to build user-friendly and effective FNS systems for assisted walking.

Miami Project scientists have extensively tested one such FNS-walking system, Parastep® 1, produced by Sigmedics, Inc., both for its functionality and its value as an exercise device. This has led to a new focus on exercise strategies for paraplegic subjects.

Brazilian racecar driver William Peetz stands and walks with the Parastep FNS-walking system

What is FNS? Functional Neuromuscular stimulation (FNS) is a rehabilitation strategy that applies low-level electrical current to the nerves that control muscles, to stimulate functional movements. "Functional" refers to the restoration of useful movements, like standing or stepping. FNS systems include (1) electrodes taped to the skin, inserted through the skin, or surgically implanted, (2) a control unit (microprocessor) to trigger and modulate the intensity of the electrical stimuli, and (3) a stimulator to generate the electrical pulse.

Practically, most people concede that FNS systems today are not designed to replace the wheelchair. FNS can be used, however, to provide greater mobility in some situations and to increase the opportunity for conditioning exercise, which is limited after SCI. Miami Project researchers Patrick Jacobs, Ph.D., John Klose, Ph.D., and the Bantle Center's staff of therapists have been exploring the potential of FNS-walking to provide effective new opportunities for exercise and conditioning in subjects with paraplegia.

In April 1994, Parastep became the first FDA-approved FNS-walking system, after a multicenter trial was successfully conducted. This system uses four channels of electrodes taped to the skin over major muscle groups in the thigh and lower back to extend the hip and knee joints (for standing). For stepping, a half-second pulse through another pair of electrodes provides stimulation below the knee to the peroneal nerve, triggering a flexor withdrawal reflex.

The system is controlled by the user, through a controller worn on a belt and buttons on the handles of a walker. It was designed for use by people with motor-complete injuries from T4-T11. Persons with higher injuries encounter problems with the hand controls and torso support. The Parastep design was based on work by Graupe and Kohn. Similar systems have been developed and tested in other laboratories, but physiological effects are likely to be comparable with similar FNS devices.

A Thorough Investigation

Jacobs and Klose performed extensive studies using Parastep, evaluating functionality based on the distance walked, pace, and length of time subjects could stand and walk. In tests conducted on 32 subjects during an 11-week training period, subjects were able to use the system to walk an average of 120 meters; some could walk up to a mile and a quarter.

The pace of FNS-walking was slow, a quarter mile per hour on average, which is much less than the 3-3.5 mph average in uninjured persons. Top speeds averaged 0.5 mph, which physiological measures showed required as much effort as running 8 mph for nondisabled subjects.

Overall, the researchers concluded that the FNS-walking was "functional" over short distances, and noted that some subjects did quite well. Still, the system is not fully functional for the needs of everyday life; imagine crossing a major intersection at one-twelfth your normal pace.

FNS-walking showed much more promise as an exercise tool. The most obvious physical effect was enlarged thigh and calf muscles, but the internal benefits were more revealing. After the training, subjects could perform for significantly longer periods during an arm-crank test before showing signs of fatigue.

At first glance, improvement in arm performance after training of the lower limbs seems counter-intuitive. A collaboration with colleague Mark Nash, Ph.D., helped explain the effect. In these studies, lowered heart rates and metabolic measures indicated that the subjects' cardiovascular systems were working more efficiently. This was attributed to improved blood flow in the paralyzed lower extremities and efficiency of the heart. "In a physical stress situation the body responds more efficiently; the heart rate is lower under the same types of stress," explained Jacobs. "All day long, the resting heart rate was decreased, which is another sign of cardiovascular training and fitness."

Loss of bone mineral density (BMD), osteoporosis, is rapid and significant after SCI. Although the reasons for this are not fully understood, it was hoped that weight-bearing, FNS-walking could improve BMD. Disappointingly, no significant change in femur density was observed after training.

Interestingly, the "rate of perceived exertion," the subjects' estimate of how hard they were working, did not accurately reflect their level of exercise intensity. Due to loss of sensation, the subjects could not judge fatigue accurately; it set in abruptly near peak levels of exertion. The studies showed that oxygen consumption and heart rate measures more closely reflected the amount of exertion during FNS-walking.

It Feels Good – Do It

Exercise has a positive psychological effect, improving self-concept and alleviating symptoms of depression and anxiety. Standing and walking has significance for subjects with SCI beyond just exercise. Evaluations before and after training confirmed the psychological benefits of FNS-walking; physical self-concept increased and depression decreased significantly.

We know exercise is good for us, but most of us still struggle to get enough of it. We all have excuses, but for persons with SCI, loss of mobility and access to opportunities for conditioning exercise are major obstacles. "As of now, the largest segment of the paralyzed population has not had access to these FNS-technologies," says Nash, who is working with Jacobs on new circuit training studies. "We are therefore examining exercise strategies that can be made available to larger numbers of individuals with SCI at an affordable cost."

After SCI, the stress on arms and shoulders caused by repetitive motions (e.g., transfers and pushing wheelchairs) can lead to overuse injuries. Exercise programs need to counteract these stresses, not exacerbate them. "Most of the movements a paraplegic experiences are downward pressing. Very seldom are they reaching up and pulling down or reaching ahead and pulling toward their bodies, so those movements are weak.

High intensity arm ergometry [arm-crank] exercise, unfortunately, can lead to shoulder injuries, plus there is going to be fatigue," says Jacobs. "You are using the same muscles over and over again and the relatively less efficient upper body muscles will fatigue before you really get a dramatic cardiovascular training benefit."

Nash and Jacobs are testing a new training program designed to provide full range of motion across all the upper extremity joints, especially movement across the scapula, to improve muscle strength and provide cardiovascular conditioning. "These exercise interventions address the medical complications that will be faced by persons aging with paraplegia," says Nash. "These include cardiopulmonary diseases and musculoskeletal dysfunction."

Circuit training involves 45 minutes of continuous exercise using a Helm multi-station machine modified to allow wheelchair access. The circuit consists of low-resistance arm cycling and five resistance exercises. Two minutes of cycling then two one-minute resistance exercises are alternated to keep heart rate elevated, work different upper body muscle groups and fully mobilize the joints.

The team has completed their first three-month trial and reported their results at international Sports Medicine meetings. Their data show significantly increased strength, less fatigue, and improved cardiopulmonary measures. "These preliminary findings suggest that chronic paraplegics alter their resting cardiac function and better adapt to acute cardiac stress following circuit resistance training," Nash noted.

"The bottom line is," adds Jacobs, that "even though the resistance training is primarily anaerobic, we saw improvements in the aerobic capacity." Many questions remain, he noted, "Is that directly related to their arms being stronger so they can continue longer before there is arm fatigue? There could be enhancements in the respiratory system or the vascular system. Is the heart pumping more blood?" Answers to these questions will help optimize exercise strategies.

Frank Diaz pulls hard at a resistance station while Charles Bradley (background) keeps his heart rate up with two minutes of arm-cycling.

Subjects have reported that feeling stronger helps them negotiate daily tasks like getting into their vans. Future plans are to extend the studies to older and female subjects, and to explore new ways to improve exercise options available to the SCI population. "The goal would be to make it applicable to many different settings. The easiest place would be in some kind of a club that would do circuit training or a rehab center that may have open hours in the evening or at lunch time," suggests Jacobs. FNS-devices like Parastep could be available in such centers, along with exercise tools for persons with tetraplegia, offering SCI persons ever more opportunities to stay fit.

We thank the Florida Brain and SCI Program, and NFL Charities, whose support makes this work possible.